Conventional voltage regulators are divided into two types: those having a circuit configuration consuming a large amount of electric current to increase power-supply rejection ratio (PSRR) and load transient response and those requiring no high-speed response and thus having a circuit configuration consuming a smaller amount of electric current. If a voltage regulator having high-speed response is employed in an apparatus, such as a cellular phone, that consumes a normal amount of electric current in an operating state and a reduced amount of electric current in a wait state such as in a sleep mode, the voltage regulator incurs a great loss in electric current consumption when the apparatus is in the wait state where no high-speed response is needed.
Accordingly, as shown in FIG. 1, a voltage regulator 101 that consumes a large amount of electric current but has high-speed response and a low-speed-operation voltage regulator 102 whose electric current consumption is controlled to a lower level are provided to be connected to a load 110 via a changeover switch 103. The voltage regulators 101 and 102 have respective output transistors 105 and 106 of different sizes, but are equal in configuration. The output transistor 105 of the voltage regulator 101 has a large electric current supply capacity.
The changeover switch 103 exclusively connects the voltage regulator 101 or 102 to the load 110 based on a control signal supplied from an external control apparatus 111. That is, when the load 110 operates with a normal amount of electric current consumption, the control apparatus 111 controls the changeover switch 103 so that the load 110 is connected to the output terminal of the voltage regulator 101.
On the other hand, when the load 110 operates with a reduced amount of electric current consumption, the control apparatus 111 controls the changeover switch 103 so that the load 110 is connected to the output terminal of the voltage regulator 102. Thus, by selectively using the voltage regulator 101 or 102 based on an amount of electric current consumed by the load 110, the amount of electric current consumed by the voltage regulators 101 and 102 can be controlled or reduced.
According to such a configuration, however, the output transistors 105 and 106 each require a large area on the chip if the voltage regulators 101 and 102 and the changeover switch 103 are formed on the same single semiconductor chip. Further, the same amount of electric current that flows through the output transistors 105 and 106 is required to flow through the changeover switch 103, so that a large chip area is required to reduce the resistance of the changeover switch 103. Accordingly, in the case of forming the voltage regulators 101 and 102 and the changeover switch 103 on a single semiconductor chip, the chip area increases to incur an increase in cost.